Thermoplastic sacks and bags are used in the packaging, transportation or storage of a great variety of materials ranging from confidential documents, powders and granules, bulky and lightweight materials, and agricultural materials such as hay and silage. The thermoplastic bags according to this invention have general applicability to such products.
Bulky, but lightweight materials such as fibreglass insulation and peat moss are generally shipped in compressed form in thermoplastic sacks. These sacks are generally known as tubular insulation sacks or bags and take the form of an extended envelope or tube sealed at one end prior to its being filled with product. For the most part these bags are produced by the commonly known in the art "blown film" process, which owes its popularity to the fact that it can be quickly and readily adapted to the production of different widths and thicknesses of continuous tubes which can then be easily cut to length and sealed at one end to produce an open top bag.
It will be readily appreciated that the thinner the film thickness (gauge), commensurate with acceptable film properties, the less the amount of thermoplastic material required. This downgauging of sack wall thickness is a most desirable industrial goal. Walls of sacks produced as tubes by the blown film process, typically, have a film thickness in the range of 3-6 mil, which is generally determined by the machine direction tensile strength necessary to handle the package weight, the film stretch resistance required to prevent expansion of the compressed product and the puncture resistance of the bag for distribution handling. The tubes from which these sacks are commonly made are produced with a bubble diameter/die diameter generally of 3:1 in order to optimize film strength properties.
The typical film manufacturing process basically comprises the step of extruding molten thermoplastic material through a circular die, typically having a 0.125 cm die gap to form a tube which is closed by passing an end through a set of nip/draw rolls. Air is introduced through the die centre to inflate the film tube to the desired width and thickness-while the speed of the nip rolls is increased to draw the film thickness down to the desired gauge. For most out-of-line bag making operations the film tube is then trim slit to the desired size and wound up into two film rolls which are then slit and passed to the converting operation of printing and bag making.
Typically, in the simplest version of the blown film process, the film tube is produced as above having a 3:1 blow ratio to provide a non-oriented film of three-times the thickness of the desired finished bag, while the film width is, typically, approximately 10 cm wider. The film tube is slit into sheeting which is passed to a machine direction orienter, where it is drawn down below the melting point of the thermoplastic material, for example, at 105.degree. C. for a linear low density polyethylene, over a narrow gap (typically 0.125 cm) between nip rolls to the desired finished bag thickness. This cold drawn film may then be manufactured into bags by methods known in the art.
While the above general method is suitable for the production of light gauge film, particularly for use in multi-ply bags, it is not generally suitable for heavy gauge film production. For example, it is difficult to produce by the blown film process linear low density polyethylene film of a thickness greater than 0.015 cm, at a blow ratio of 3:1. This is because the hot melt strength of these films is insufficient to support the bubble, whereby the latter droops causing gauge and width control to be lost.
In view of the usefulness of the blown film process for producing film in tubular and, subsequently, sheet form there is a need to extend this usefulness to enable heavy gauge oriented film and bags made therefrom to be readily manufactured.
It is an object of the present invention to provide an improved method for making heavy gauge oriented film involving the blown film process.
It is a yet a further object of the present invention to provide heavy gauged oriented film.
It is a yet further object of the present invention to provide improved bags comprising heavy gauge oriented film.